This invention relates to continuous casting systems, and more particularly to such a system in which the mold is oscillated during casting. Still more particularly, the invention is directed to a method and apparatus for monitoring the friction between the mold and the mold contents during oscillation of the mold.
In a conventional continuous casting system, a plug or "dummy bar" is inserted through the bottom of an open-bottomed mold, and molten metal is then poured into the mold. The mold sidewalls are typically water-cooled, and the molten metal cools significantly faster around the periphery of the mold. As the metal begins to harden, a skin develops around its periphery and the plug is slowly withdrawn from the bottom of the mold. As the strand is withdrawn from the bottom of the mold, molten metal is continuously poured into the top of the mold, and a long strand of casting can be obtained from a relatively small mold.
In order to insure that the skin does not adhere to the mold surface, the mold is typically oscillated in a vertical direction at a relatively low frequency, e.g. 3 Hz. The mold is also typically coated with a flux or oil to decrease as much as possible the friction between the strand and mold. Despite these friction-reducing measures, it is still possible for the skin to adhere to the mold surfaces. This will result in a degradation of the surface of the cast product, and worse, may result in a "breakout" where the skin tears and the internal molten metal escapes as the strand is pulled out of the bottom of the mold. It is therefore imperative that the friction between the mold and skin be closely monitored.
The simplest method of monitored the friction between the mold and casting skin is to monitor the load on the mold oscillating mechanism, and a number of such techniques have been developed. Grenfell, in his British Patent Specification No. 1,556,616, discloses an arrangement wherein load transducers are provided between the mold and the support table to weigh the mold so that both the static weight of the mold and the apparent weight of the mold during withdrawal of the strand can be determined and utilized to establish the frictional force. In the Grenfell monitoring system, the waveform of the load signal is compared with a reference waveform developed during a previous casting operation, and corrective action is taken whenever the present waveform exceeds the reference waveform in either direction. The Grenfell system can be unsatisfactory in that factors other than the mold friction may cause transients in the load signal, and these transients will exceed the reference waveform and result in an erroneous diagnosis of a high friction condition. Further, the waveform monitored by Grenfell is the total load waveform consisting of both a static offset portion and a dynamic portion. If some change occurs in the static offset portion and thereby raises or lowers the overall level of the load signal, a high friction condition may be diagnosed even though no such condition has occurred.
Another monitored technique is disclosed in European Pat. No. 44-291. In this system, a load cell is located at each of the four corners of the mold table and the outputs of the four load cells are summed to obtain a total force signal. The static weight of the mold is then effectively subtracted from the total force signal by zeroing the system at rest, and an accelerometer-generated signal allegedly corresponding to the dynamic mass of the mold is then further subtracted. The final result is a signal roughly indicative of the friction between the casting and mold sidewalls. This final signal is then monitored to determine when a high friction condition occurs.
The system described in the European Patent suffers from disadvantages similar to those in the Grenfell system. Transients occurring in the load cell outputs, which transients may not be due to friction between the mold and strand, will be passed through to the final signal and will result in erroneous diagnosis of an excessive friction condition. Further, conditions other than mold friction may change during the casting operation to raise or lower the overall level of the load signal. The raising or lowering of the overall level of the load signal, as opposed to the level of merely the dynamic portion of that load signal, cannot be distinguished in the European Patent system.
A still further friction monitoring technique is disclosed in U.S. Pat. No. 3,893,502 to Slamar. According to the Slamar technique, the armature current of the motor used to oscillate the mold is monitored. The armature current is integrated to average the current value over some predetermined interval, and from this average value is subtracted a substantially DC signal indicating the average current value during free-running operation (i.e. during oscillation of an empty mold). The difference will then indicate the amount of the load signal which is due solely to mold friction.
Due to its integration of the armature current, the Slamar technique will be less prone to transient-caused errors than the systems described above. However, it is similar to the above-described system in its inability to differentiate between changes which occur in the overall level of the load signal and those which occur in the dynamic portion of the signal.
While the overall load signal, including both the static and dynamic portions thereof, is important in monitoring the mold friction, there are some instances where it would be highly desirable to be able to distinguish between changes which occur in the static offset portion of the signal and changes which occur in the dynamic portion of the signal. For example, this may be particularly important in determining the effects of certain operating parameters on the mold friction, such as the composition and/or quantity of the mold flux or oil, the liquid level within the mold, the oscillator speed and stroke, and the degree of mold taper. The above-described systems will all be severely limited in their respective abilities to provide accurate information as to the precise effect which the changing of any one or more of these factors may cause.